Thulsadoom

I don't understand why bicycles have big front chainrings and smaller cogs in the rear.(typically)

It seems to me that if you are in a gear in which you're in a 39 front chainring and a 20 rear cog, then it should be essentially the same gear as being in a 42 front chainring and a 23 rear cog, is that not correct? If you look at an old singlespeed bike, or a fixie, the front ring is always significantly larger than the rear cog. When you consider chain wrap and how chains and sprockets/gears wear out when they are put under stresses associated with not having many teeth engaged when under power, doesn't it seem like it would make more sense to have the same sized sprockets on front and back to more evenly space out the load?

Even with cassettes and derailleur bikes it seems like the front chainrings are generally larger than most or all of the cassette cogs.

Motorcycles are opposite and typically have a smaller front drive sprocket and a larger rear wheel pulley, so I can't figure that out either.

I'm sure there's a simple explaination for this, can someone point me in the right direction?

tsl

Sheldon Brown's Gear Calculator

Bicycles have a long-stroke, low-RPM, high-torque, narrow-band, low-horsepower motor.

Motorcycles have a short-stroke, high-RPM, high-torque, wide-band, high-horsepower motor.

This gives the transmissions differing requirements.

CraigB

Could be too, that early derailleurs worked better when they had compact bodies and cages, handling changes between relatively small cogs that were only incrementally different in size from their neighbors.

Just a guess.

Retro Grouch 

It's carryover from the days when bicycles had big front wheels and tiny rear wheels.

BlazingPedals

If you're asking why front chainrings are 39T instead of 44T, the answer is "marketing." If you're asking why they aren't 13T, then you need more help than you're gonna get here.

AndreyT

Chainrings and cogs serve the same purpose the car gearbox serves: the allow you to redistribute the balance between torque and speed. When you need more torque, you can get it at the expense of less speed. When you need more speed you can get it at the expense of less torque. Typically, the "averagely optimal" everyday balance between torque and speed in a bicycle calls for a bigger chainring and smaller cog (for the traditional crank length and wheel diameter).

If you build some sort of unorthodox bike, like a bike with unusually large wheels (like 2x the diameter of normal wheels), then the balance will shift sufficiently to call for a larger cog.

Er.. No. 39/20=1.95, 42/23=1.82. These ratios are sufficiently different.

Although I'm not sure what point you are trying to make with these examples. In both of your examples the chainring is still almost 2 times larger than the cog.

Because for the given crank length and wheel diameter this chainring and this cog provide the "optimal" balance between torque and speed.

That would result in a bike that is biased heavily towards torque. It will be able to slowly climb steep hills with relative ease, yet it will provide no speed. I.e. in order to make that bike ride at 10mph you'd have to frantically crank your pedals at crazy and unrealistic cadence. That would be extremely inefficient and impractical. More precisely, it would have practical value when you need to climb a steep hill. But in everyday riding on a horizontal road surface you'd end up spinning the pedals like a bat out of hell and yet only get to 5 mph or so. People generally don't want to ride at 5 mph on horizontal surfaces. People want to be able to achieve 20 mph, 30 mph and more. For that you need gearing that is more biased towards speed. And that requires large chainring and small cog.

Again, if you build some unusual bike with wheels that are 2 times larger in diameter than "normal" wheels, then the optimal ratio between chainring and cog diameter might get them very close to each other. If you build a bike with wheels that are 3 times larger in diameter than "normal" wheels, you might end up needing a cog that is larger than the chainring.

Motorcycle engine works at very high RPMs (extremely high RPMs, actually) while producing relatively small torque, which means that motorcycle's transmission system has to be tailored towards converting speed into torque. For this reason for motorcycles the optimal torque-speed balance calls for smaller chainring and larger cog.

A bicycle uses an "engine" of a completely different nature. A human being as an engine works more efficiently at relatively low RPMs, while producing relatively high torque. This means that bike's transmission system has to be tailored towards converting torque into speed. For this reason for bicycle the optimal torque-speed balance calls for larger chainring and smaller cog.

Well, are you familiar with the concepts of "gearbox", "gearing"? That's the primary thing about the chain drive. The purpose of the bicycle chain drive is not even remotely limited to simply transferring the torque from the crankset to the rear wheel. The purpose of the bicycle chain drive is to change the balance between the torque and speed to provide the most efficient/comfortable ride (torque vs. speed balance) for the given riding conditions.

fietsbob

Or more simply its like moving the fulcrum under a lever..
Think of a teeter-totter is a 1:1 gear.

Catapult flings stuff far because the leverage advantage is there.

Monster Pete

It's not correct. You divide the chainring by the sprocket to get a gear ratio. For example, a 38t chainring coupled to a 19t sprocket gives you a 2:1 ratio, the same as 42/21. Higher ratios give you more speed, but less torque.

This combination gives you a fairly high gear ratio, which is what you want if you only have one gear. This allows you to pedal at speed without having to pedal like crazy. The same sized gears would give you a 1:1 ratio, which might be good for rock-crawling but little else.

Motorcycle engines are capable of high revs but low torque, the exact opposite of a human cranking pedals. The low gear ratios of a motorcycle convert a high speed, low torque output to a low speed, high torque one.

Thulsadoom

After reading through this stuff, I realize now that it was kind of a dumb post. Maybe I shouldn't post in the morning when I'm on coffee....

If you wanted to equal out the cogs, say a 30 in the front and a 30 in the back, it would be a tiny gear. Unless, like AndreyT said, you had a huge rear wheel, or you could pedal at 200 rpms with no problem. A 20 in the front and 20 in the back would be tiny, a 50 and a 50 also. Somehow I confused myself and didn't realize that bicycles are geared (given wheels that are 27 inches or so in diameter) for humans who pedal at around 80 rpms comfortably and have a relatively similar power band. That's the way I see it now, and it makes sense...to me at least.

Thanks for the replies.

fietsbob

A 1:1 chain drive is how those 6 foot tall Unicycles work.

and artistic cycling bikes to do wheelies and handstands on the bike
at the same time.